Sometimes, visitors need that little impetus to get them to do something. A countdown timer gives that pyschological push that some people need to get them to buy your product, sign-up to your newsletter or click that 'like' button.
Don't forget though, the countdown should typically be above the fold.
Editing a section is very easy. Yes, you'll have to get your hands dirty using the Liquid/HTML block and in this case a little bit of JavaScript, but don't worry, the code is well documented so it's easy to understand, change and revert.